Tuesday, 7 April 2015

#Arctic Moisture on the Move

Arctic sea ice—frozen seawater floating on top of the Arctic Ocean
and its neighboring seas—grows in the fall and winter and melts in the
spring and summer. Since 1978, satellites monitoring this annual growth and retreat have detected an overall decline in Arctic sea ice.

Scientists such as NASA’s Linette Boisvert want to know how this
decline is contributing to a warmer and wetter Arctic. One way to find
out is by looking at the energy balance
at the surface. Areas of ice-free ocean absorb more heat from the Sun
and become warmer, increasing humidity near the surface. When the
humidity at the surface is higher than that of the overlying air, the
moisture is released into the atmosphere. In its vapor form, this water
is a greenhouse gas that can lead to further warming and ice loss.

The map above, produced with data from the Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua
satellite, represents the vertical transport of moisture over the
Arctic on June 21, 2014. Orange and red areas show where moisture is
leaving the surface and entering the atmosphere (evaporation); blue
areas are where moisture is moving from the atmosphere to the surface.
The rate at which this occurs is called the moisture flux.

Data for this map were acquired on the summer solstice, after the sea
ice had started its annual retreat toward its minimum extent (usually
reached in September). The transition between sea ice and ocean water is
visible where the moisture flux switches from negative (blue) over the
solid sea ice pack to positive (red) over ice-free waters.

The second image shows a close up view of the Greenland Sea (within
the box outlined in the top image) that was acquired on the same day by
the Operational Land Imager (OLI) on Landsat 8.
In this natural-color image, sea ice is less concentrated and exposes
some of the dark, relatively warm ocean to the overlying air.

According to Boisvert, it’s possible that cold, dry air from the
north or from Greenland moved over this area of broken ice. That, in
turn, could have caused the evaporation that appears orange in the top
image. The extra water vapor in the air, combined with the cold air,
would condense into the low, transparent clouds that show up in the
second image.

This cloud type has implications for the Arctic’s energy balance. The
clouds are transparent enough to let some shortwave radiation through
to the surface. But they also might prevent some longwave radiation from
leaving the surface, Boisvert said. “This combination could cause more
warming at the surface and increased sea ice melt.”